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As reported in many places (BBC News and The Register amongst others), Shell have stopped accepting Chip and PIN transactions at all 600 of their directly owned petrol stations in the UK. It is reported that eight arrests have been made, but only a few details about the modus operandi of the fraudsters have reached the media.

They have used an old style skimming device. They are skimming thecard, copying the magnetic details – there is no new fraud here. Theyhave managed to tamper with the PIN pads. These pads are supposed tobe tamper resistant, they are supposed to shut down, and so that has obviously failed.

It is not clear from the information that has been released so far whether the “magnetic details” were obtained by the attackers through reading the magnetic stripe, or by intercepting the communication between the card and the terminal. Shell-owned petrol stations seem to use the Smart 5000 PIN pad, produced by Trintech. These devices are hybrid readers: it is impossible to insert a card (for a Chip and PIN transaction) without the magnetic stripe also passing through a reader. With this design, there seem to be two possible methods of attack.

A hardware attack. Given the statement that “[the attackers] have managed to tamper with the PIN pads”, perhaps the only technical element of the fraud was the dismantling of the pads in such a way that the output of the magnetic card reader (or the chip reader) could be relayed to the bad guys by some added internal hardware. Defeating the tamper-resistance in this way might also have allowed the output from the keypad to be read, providing the fraudsters with both the magnetic stripe details and a corresponding PIN. It seems fairly unlikely that any “skimming” device could have been attached externally without arousing the suspicion of consumers; the curved design of the card receptacle, although looking ‘suspicious’ in itself, does not lend itself to the easy attachment of another device.

A software-only attack. The PIN pads used by Shell run the Linux kernel, and so maybe an attacker with a little technical savvy could have replaced the firmware with a version the relays the output of the magstripe reader and PIN pad to the bad guys. The terminals can be remotely managed  a successful attack on the remote management might have allowed all the terminals to be subverted in one go.

The reaction to the fraud (the suspension of Chip and PIN transactions in all 600 stations) is interesting; it suggests that either Shell cannot tell remotely which terminals have been compromised, or perhaps that every terminal was compromised. The former case suggests a “hardware attack”; the latter a (perhaps remote) “software attack”.

Even if the only defeat of the tamper resistance was the addition of some hardware to “skim” the magstripe of all inserted cards, corresponding PINs could have been obtained from, for example, CCTV footage.

Attacks like this look set to continue, given the difficulty of enabling consumers to check the authenticity of the terminals into which they insert their cards (and type their PINs). Even the mythical tamper-proof terminal could be replaced with an exact replica, and card details elicited through a relay attack. Members of the Security Group have been commenting on these risks for some time, but the comments have sometimes fallen on deaf ears.